This study examined the magnitude of release of fecal coliform indicator bacteria in surface runoff from pasture amended with dairy manure. The experiment was conducted at Southeast Research Station in Franklinton, Louisiana between March and June, 2001. The experiment was designed to compare the effects of two methods of manure application on fecal coliform release. Manure application simulating natural deposition by cattle (Treatment A) was compared to that of land application of manure as fertilizer (Treatment B). Application of inorganic fertilizer was used as the control. Each Treatment was applied to three field plots. Simulated rainfalls were conducted on plots within hours of initial manure application and approximately 2, 7 and 14 days following initial manure application. This sequence of manure application and subsequent rainfall events was repeated three times. Runoff samples were analyzed for fecal coliform analyses using Membrane Filtration (APHA, 1995). Also, the IDEXX Quanti-Tray-Colilert total coliform method was experimentally modified to determine its validity for enumerating fecal coliforms.

Fecal coliform results indicated that Treatment B had significantly higher fecal coliforms in runoff than Treatment A. Fecal coliforms in runoff from manure-amended plots were well above the 200 CFU/100 ml recreational water use standard, with typical counts ranging from 1,000 to 1,000,000 CFU/100 ml for Treatment A and 10,000 to 10,000,000 CFU/100 ml for Treatment B. Fecal coliform concentrations in the runoff from the second or third rainfall event were often higher than those from the initial rainfall after manure application. Both manure Treatments had significantly higher fecal coliforms in runoff than the control. Typical counts from the control plots ranged from 10 to 1,000 CFU/100 ml.

The modified Quanti-Tray method produced results that were significantly correlated with those of membrane filtration for both manure Treatments. However, the control Treatment did not show similar correlation. In addition, paired t-tests indicated that the results of the Quanti-Tray method were not significantly different from those of membrane filtration for both manure Treatments. However, the results of the two methods were significantly different for the control Treatment.